In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. A transceiver module generates amplitude modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. The transceiver module includes a transmitter side and a receiver side. The transmitter and receiver sides may be controlled by a transceiver controller. The transmitter side typically includes a laser driver and a laser diode. The laser driver outputs electrical signals to the laser diode that modulate the optical power of the laser diode in accordance with a particular modulation scheme (e.g., a binary encoding scheme) to produce output power levels corresponding to logic 1s and logic 0s. Other modulation schemes exist, including schemes that use modulators that are separate from the laser diode and that have other than two output levels. An optical system of the transceiver module focuses the coherent light produced by the laser diode into the end of a transmit optical fiber.
On the transmitter side, a monitor photodiode typically monitors the output power levels of the laser diode and produces respective feedback signals that are fed back to the transceiver controller, which processes them to ascertain the average output power level of the laser diode. The transceiver controller outputs control signals to the laser driver to cause it to adjust the bias current signal output to the laser diode such that the average output power level of the laser diode is maintained at a relatively constant level. A receive photodiode on the receiver side receives an incoming optical signal output from the end of a receive optical fiber. An optics system of the receiver portion focuses the light output from the end of the receive optical fiber onto the receive photodiode. The receive photodiode converts the incoming optical signal into an electrical signal, which is then processed by other circuitry of the receiver side, such as amplification circuitry or clock and data recovery circuitry, for example.
Various types of tests are performed on the transmitter of the transceiver module to determine if it will operate as intended and provide a sufficient signal for a receiver to recover. For example, a variety of tests are often performed on the optical waveform using a communications signal analyzer (CSA). One such test is known as an eye mask test. An eye mask is a template that defines display regions where the waveform is permitted and regions where the waveform should not occur. To perform an eye mask test, a pattern generator of the CSA generates bit sequences, which are used to modulate the laser diode of the transmitter. Optical-to-electrical circuitry of the CSA converts the optical waveform into an electrical waveform. A sampling oscilloscope of the CSA repetitively samples the electrical waveform and displays a superposition of digitized time-domain representations of the waveform on the display monitor of the CSA. The displayed superimposition of the waveform is commonly referred to as an eye diagram due to the fact that it resembles a human eye. An eye mask can also be displayed on the display monitor. By viewing the displayed eye diagram and its relationship to the displayed eye mask, the engineer or technician performing the analysis can, at a glance, partially determine whether the transmitter will perform as expected.
In general, an eye mask represents a combination of requirements, dynamic response characteristics of overshoot and ringing that are not otherwise specified, as well as transition times and jitter attributes that may have additional specifications, for the transmitter output signal. An eye mask typically comprises three polygons: one above the eye diagram, one inside of the eye diagram and one below the eye diagram. The three mask regions constrain the dynamic response, overshoot and ringing, of the transmitter to that tolerated by receiver. The inner polygon defines the open eye requirements and constrains the transitions times, jitter and separation between signal levels such that the receiver can distinguish between them. A variety of eye masks have been defined by various communications standards to ensure that the corresponding signal has acceptable quality. When an eye mask test is to be performed, the person performing the test uses controls on the signal analyzer to select the appropriate mask to be applied to the signal waveform being measured. During testing, attributes of the transmitter can be ascertained based on whether the eye diagram extends into the eye mask regions and based on how near the eye diagram is to the eye mask regions, which is commonly referred to as the mask margin.
Eye mask tests are typically performed on parts after they have been manufactured, but prior to the product being shipped to the customer. Using an eye mask defined by the appropriate standard, the user is able to determine whether the corresponding eye diagram extends into one of the mask regions defined by the eye mask, which is commonly referred to as a “hit” on the mask. For some standards, if the eye diagram hits one of the eye mask regions, the transmitter is deemed to be noncompliant with the associated standard. In general, if the transmitter is deemed to be noncompliant with the associated standard, the part containing the transmitter is deemed to be unsuitable for shipment to the customer. Thus, a hit on the eye mask region results in reduced manufacturing yield. In addition to the requirement that there be no hits on the eye mask, customers often require that the manufacturer determine the mask margin and ship parts to the customer that have a particular minimum mask margin. For example, a particular customer may require a mask margin of at least 10%.
Eye mask tests generally are not used to perform quantitative analyses of signal quality attributes such as signal rise and fall times and jitter or engineering verification of a product or detailed product characterization. Rather, eye mask tests currently used are best suited for determining if high probability deterministic characteristics of the signal are compliant with the applicable communications standard. Under suitable conditions, eye masks can also be used to evaluate lower probability characteristics of the signal. An eye diagram is generally deemed to be compliant with the applicable standard if the eye diagram does not “hit” a mask region.
A variety of quantitative tests are used for particular transmitter attribute measurements. FIG. 1 illustrates a table that contains a list of transmitter attributes including the eye mask definition that are required by two different communications standards, namely, the 10 gigabit per second (Gb/s) Ethernet standard P802.3ae and the Fibre Channel standard FC-PI-4. It can be seen from the table that these standards require a variety of tests to be performed in addition to performing the eye mask test. Performing these additional tests consumes time and effort, and generally requires the use of additional test equipment and additional fixtures on the test equipment. Performing these additional tests also increases overall costs.
One of the problems associated with the current eye mask testing methodology is that it is possible for a transmitter that actually performs satisfactorily to fail the eye mask test. In other words, when performing eye mask testing using the current methodology, one or more hits by the eye diagram on the eye mask defined by the applicable communications standard equates to a finding that the transmitter is noncompliant with the standard. When this happens, the transmitter is generally deemed unsuitable for shipment to the customer. In addition, with the current eye mask testing methodology, even if there are no hits on the eye mask, the transmitter may be deemed unsuitable for shipment to the customer if the eye diagram is compliant with the mask dictated by the standard, but fails to meet a margin to the mask (the “mask margin”) dictated by customer needs. In either case, the determination that the transmitter is unsuitable for shipment results in reduced manufacturing yield, which increases manufacturing costs.
However, even if the eye diagram hits on the applicable eye mask, or has a mask margin that is less than that required by the standard, this does not necessarily mean that the transmitter does not operate satisfactorily. For example, if the transmitter provides more than the minimum required output modulation amplitude than that required by the applicable standard, it is quite possible that the transmitter will operate satisfactorily despite the eye diagram appearing partially closed. Nevertheless, using the current eye mask testing methodology, such a transmitter would be deemed to be unsuitable for shipment to the customer.
It would be desirable to provide an eye mask test that would eliminate the need to perform separately at least some of the tests that are currently performed to measure many of the attributes listed in the table shown in FIG. 1. Eliminating the need to perform many of these additional tests would reduce the time, effort and cost associated with testing, thereby reducing the amount of time that is required to make the product available to the customer as well as the costs associated with manufacturing and testing the transmitters. It would also be desirable to provide an eye mask test that reduces the possibility that a transmitter may fail the eye mask test even though the transmitter operates satisfactorily.